1. Field of the Invention
The present invention relates to an improvement on a light emitting device employing, for example, electroluminescence (EL), and a method of extracting light using the same, and more particularly to a technique for improving the efficiency of extracting light from the light emitting device.
2. Description of the Related Art
EL light emitting devices (panels) using inorganic EL, organic EL, etc. are widely used in display devices and the like. In the inorganic EL light emitting device, a light emitting layer formed of ZnS:Mn is used, and 180 to 200 V of alternating voltage is applied to a three-layer construction in which the light emitting layer is sandwiched between insulating layers formed of SiNx or the like. Concurrently, electrons collide into Mn atoms to be excited, allowing orange light to be emitted therefrom. On the other hand, the organic EL light emitting device is basically of a two-layer construction including a hole transporting layer and a light emitting layer. Electron holes injected from an anode and electrons injected from a cathode recombine in the light emitting layer, generating molecular excitons therein. Here, fluorescent is used that is generated when the excited excitons return to a ground state. Since the light emission is of a charge injection type, the organic EL light emitting device is also referred to as an organic light emitting diode.
FIG. 4 shows a basic construction of the organic EL light emitting device. The EL light emitting device shown in FIG. 4 comprises a glass substrate 12, a linear ITO transparent electrode (anode) 14, a first insulating layer 16, a hole transporting layer 18, a light emitting layer 20 capable of emitting light through electroluminescence, and a linear backside electrode (cathode) 22. Note that, the direction of viewing is from the bottom of the diagram.
In the EL light emitting device shown in FIG. 4, electrons and electron holes (positive holes) are injected from the pair of electrodes, namely the cathode 22 and the anode 14, into the light emitting layer 20. The electrons and the holes recombine in the light emitting layer 20, and when the molecules return from an excited state to a ground state, light is generated. Then, the light (normally, a portion of the light) generated in the light emitting layer 20 penetrates through the transparent anode 14 to be extracted on a light extraction side.
Note that, for the details of the EL light emitting device (panel) described above, reference may be made to “Electronic Information Communication Handbook” (Ohmsha, published in 1998) Section 5, Subsections 5 to 12, on the page of “Electronic Display and Printer”, for example.
Incidentally, in the EL light emitting device having the above-mentioned construction, the light emitting layer 20 and the air have different indexes of refraction from each other. Therefore, conventionally, there is a problem that only approximately 20% of the light (luminescence) generated in the light emitting layer 20 can be extracted.
The cause of this is that, in the process of penetrating through the ITO transparent electrode 14 and the glass substrate 12, the light (luminescence) generated in the light emitting layer 20 is refracted again and again, and a total reflection occurs inside the glass substrate 12, whereby the light is often trapped therein. Specifically, when the organic layer is used as the light emitting layer 20, its refraction index is approximately 1.7, the refraction index of the glass constituting the glass substrate 12 is approximately 1.5, and the refraction index of the external air is approximately 1.0. Therefore, only the light emitted at emission angles within approximately 38° (which corresponds to a kind of critical angle) with respect to a line set perpendicular to the glass substrate 12 is extracted to the exterior.
When this is calculated in terms of solid angles, only approximately 22% out of all the generated light can be extracted. This means that approximately just under 4/5 of all the generated light is being lost without being used.
In response to this, a research group of Kyushu University and the like have attempted an experiment in which a silica aerogel layer having a refraction index of 1.03, which is close to that of air, is inserted between the glass substrate and the ITO transparent electrode to improve the light extraction rate. However, although decent results are obtained by this method, control of the thickness of the light emitting layer and other aspects require sophisticated optimization techniques for the EL light emitting device, and thus the method has a critical problem that it cannot be realized easily.